Address: 23 Rue de Sévigné, 75003 Paris
Website: (in English) http://carnavalet.paris.fr/en/museum-carnavalet
This is an excellent museum which traces the history of Paris with dozens of rooms filled with art and artefacts, furniture, costumes and decorative items from ancient times to the present.
The gardens and buildings in which the museum is housed are incredibly beautiful. You can spend hours here and, almost unbelievably, it's free.